.3 Disabling Attic Exhaust Fan
The third retrofit involved turning off the attic exhaust fan. Average cooling energy consumption (based on an eight-month cooling season) decreased by an additional 36%. The 14.3 kWh/day saved breaks down to 11.2 kWh/day from the exhaust fan motor and 3.1 kWh/day from a reduction in air conditioning energy. Savings over an eight month cooling season are projected to be 3208 kWh or $289. Assuming a service call cost of $50 for turning off the attic fan, the simple payback would be after two months. Peak electric demand (including attic fan power of 0.47 kW) was reduced by 21% from 3.4 kW to 2.7 kW.
Impacts of Disabling Attic Fan on Ventilation, Humidity, and Building Pressure
Turning off the attic fan produced dramatic impacts upon the office. The large driving force (pressure differential) causing air conditioned air to be sucked out of the occupied space and hot humid air to be drawn into the occupied space is gone when the fan is off. Table 3 is located in the Conclusions section of this paper and summarizes measured ventilation rates, relative humidity, and building pressures during initial, duct repair, and attic fan off monitoring periods. The natural air change rate (all mechanical equipment off) is not shown in Table 3, but was measured on a couple testing days and averaged 0.17 ach.
Building pressure decreased from -0.064 in.WC (-15.9 Pa) to -0.0024 in.WC (-0.6 Pa). This resulted in a large drop in the ventilation rate and the indoor relative humidity. The 24 hour average relative humidity levels plummeted from 77% to 61%. Relative humidity levels are shown in Figure 4 during a composite 24 hour day consisting of one business week of data. Each hour represents the preceding hour of gathered data. For example, hour 2 represents data gathered beginning at hour 1 and ending at hour 2. Non-business hours from 12 AM to 7 AM show the humidity drop from an average of 83% when the fan is on, to an average of 65% when the fan was off. During business hours from 11 AM to 5 PM, the relative humidity drops from an average of 68% when the fan is on to an average of 55% when the fan is off. The building ventilation rate decreased from 0.79 air changes per hour (ach) to 0.33 ach. Due to diminished ventilation, the peak carbon dioxide concentration increased from an average 614 ppm to 1054 ppm during weekday hours of 3 to 5 p.m. Figure 5 shows carbon dioxide concentrations during a composite 24 hour day consisting of one business week of data.
Figure 4 Daily composite of indoor relative humidity before and after attic fan was turned off.
Figure 5 Daily composite of carbon dioxide levels before and after attic fan was turned off.
Measured ventilation rates and carbon dioxide concentrations indicate that this office needs additional ventilation after duct repair and with the attic fan off to be in accordance with ASHRAE 62-1989. Typical occupancy during normal business hours is eight adults. According to ASHRAE 62-1989, the desired amount of ventilation air would be 20 cfm (9.4 l/s) per person which totals 160 cfm (76 l/s) for eight people. Infiltration testing by means of tracer gas decay indicated a total of only about 58 cfm (27 l/s) or just over 7 cfm (3.3 l/s) per person. A suggestion was made to the building manager to contact a qualified air conditioning and ventilation contractor to increase the ventilation rate, but no changes were desired to be made by the business during the time that monitoring was conducted. It was not desirable to turn on the attic fan because it increased the relative humidity significantly. A discussion of how the ventilation could have been provided follows.